This invention relates generally to hypodermic techniques for obtaining blood samples, more particularly to a hypodermic syringe assembly for drawing blood from a patient, especially a syringe useful for drawing samples from the patient's arteries for blood gas analysis or other testing.
Various apparatus and methods for taking blood samples from patients have been previously known. Such samples are normally taken by means of a syringe which includes a cylindrical tube having a piston therein which, when pulled by an operator creates a suction force drawing blood into the tube through a nozzle coupled to a hypodermic needle. Many of the tests are performed on blood which is obtained from the veins of the patient. However, an increasingly, important method of determining the medical status of a patient is the obtaining of arterial blood samples, particularly for testing the blood for its content of various gases. Such samples are tested for the partial pressure of oxygen, the partial pressure of carbon dioxide, the pH of the blood, the electrolyte balance, and various other tests known in the art.
The syringes previously used in obtaining arterial blood samples have been glass syringes, in which the cylindrical body is made of glass and the piston is a ground glass rod which closely fits within the cylinder. Generally the technique for taking samples with such devices comprises as a first step the drawing of an anticoagulant solution, such as sodium heparin, into the syringe. This material also acts as a lubricant for the walls so that the glass piston may move relatively freely within the cylinder. The syringe is inverted and all air is expelled from the chamber and needle, along with the bulk of the anticoagulant solution, which is normally far in excess of the amount needed for the blood sample. It is extremely important that all air be expelled from the syringe, since one of the tests performed is the measurement of the amount of oxygen present in the blood, and even minute contamination with air will prevent accurate measurement of that amount. After suitable preparation of the patient, the hypodermic is inserted into the artery, and blood is either forced into the syringe by the pressure of the blood in the artery, or is drawn into the syringe by withdrawing the piston. One advantage of the glass syringes previously used is the ease with which the piston may be moved within the lubricated chamber. The glass piston is ground to very close tolerances, so that it is sufficiently close to the syringe wall to prevent leakage, but sufficiently far away to allow formation of a thin film of the anticoagulant. Even very low blood pressures are usually sufficient to enter the syringe and force the glass piston backwards without any aid from the person taking the sample. Upon entry into the syringe the blood mixes with whatever anticoagulant solution remained in the needle and tip of the syringe after the excess had been expelled.
The glass syringe previously used have suffered from a number of disadvantages. They are expensive, since the grinding requires close tolerances, in the order of 0.00007 inches clearance between the piston and the cylindrical syringe body. They are easily breakable, which is especially costly after the sample has been taken. The glass piston and the glass barrel of each syringe must commonly be matched during the grinding by the manufacturer, since variations in grinding from one piston to another may be sufficient to permit leakage of air or other material around the piston, which will contaminate the sample. Thus the cylinders and pistons cannot easily be individually mass produced, since the pistons often cannot be satisfactorily interchanged one with another in any given cylinder, as pointed out in U.S. Pat. No. 2,419,401 to Hinds. Further, because of the easy movement of the glass plunger in the cylinder, the plunger falls out of the cylinder of its own weight, and normally breaks on the floor, unless the syringe is carried needle end down. Special metal holders for the glass cylinder have been used to prevent this problem.
Attempts have been made to avoid these disadvantages by either manufacturing both the cylinder and the piston out of materials other than glass, such as plastics, or by using glass cylinders with plastic plungers or pistons. However, these attempts have not produced an acceptable product. In order to prevent leakage around the piston, these devices depend upon the use of a compressible tip on the end of the piston adjacent the hypodermic needle, which tip generally has a number of ribs which are larger than the inside diameter of the cylinder in their uncompressed state. The type of seal formed by such tips, however, with the materials previously used, has made the movement of the piston within the cylinder more difficult, thus normally requiring manual withdrawal of the piston to obtain the blood sample. The handling of the syringe which is involved when manual withdrawal of the piston is required may cause traumatization of the artery from which the blood is being taken. A major problem has been the fact that when pressure is applied to expel the excess anticoagulant solution, the compressible tip on the end of the piston compresses and deforms against the end of the cylinder. When the piston is released prior to the insertion of the hypodermic needle into the artery, the pressure on the compressible tip is also released, which causes the piston to move back slightly, drawing a small amount of air into the tip of the hypodermic needle. Since the samples which are drawn to test for the amount of oxygen and carbon dioxide in the blood, are very small, e.g., 2, 5 or 10 ml, even minute amounts of oxygen leaked into the sample have potentially adverse affects on the results obtained. The compressibility of the plunger tip also causes nonuniformity in the amount of anticoagulant left in the tip of the syringe and hypodermic needle. As can be readily appreciated, the amount left will depend upon the amount of pressure used to expel the oxygen and excess antiocoagulant, since greater pressure will compress or distort the compressible plunger tip to a greater degree, thus expelling more anticoagulant. If too little anticoagulant solution remains to be mixed with the blood, the blood may coagulate prior to testing and thus adversely affect the results obtained. If, on the other hand, too much anticoagulant solution is left in the syringe, its presence will also adversely affect the test, as is known in the art.
Accordingly, an object of the present invention is to provide a simple, inexpensive blood sampling syringe, particularly one suitable for taking arterial blood samples, which avoids the difficulties previously encountered with the glass syringes used for such purposes, and yet avoids any contamination of the sample which will interfere with the results obtained. It is a further object of the invention to provide such a syringe which is adapted to prevent air being sucked into the hypodermic needle when the plunger is released prior to taking the sample. It is a further object of the invention to provide such a syringe which is adapted so as to supply a uniform amount of anticoagulant solution to the blood sample being taken. It is a further object to provide such a syringe having an easily movable piston whereby the possibility of traumatization of the patient is minimized. It is a further object to provide a syringe having all of these advantages which is simple and inexpensive to manufacture, and thus of low cost to the patient, and extremely simple to operate in a manner which gives uniform and representative results.
Other objects and advantages of the present invention will be apparent from a reading of the present specification, or from the practice of the invention herein disclosed.